Field of the Invention
The present invention concerns a method and a magnetic resonance system in order to separately determine the T1 time of water and the T1 time of fat on a voxel-by-voxel basis of a predetermined volume segment.
Description of the Prior Art
According to the prior art, it is known to produce a separate of water signals and fat signals with a technique known as the Dixon method. The Dixon method utilizes the different resonance frequencies of fat and water. Due to these different resonance frequencies, the phase relationship between the water magnetization and the fat magnetization is dependent on the echo time (time interval between the RF excitation pulse and the acquired echo). For example, if a first echo time is now chosen so that the water magnetization and the fat magnetization are in phase, and a second echo time is chosen so that the water magnetization and the fat magnetization have a phase relationship of 180° (opposed phase), the following Equation (1) then applies for the magnetization M1 acquired at the first echo time and the following Equation (2) applies for the magnetization M2 acquired at the second echo time.M1=W+F  (1)M2=W−F  (2)wherein W is the water magnetization (i.e. the magnetization caused by the water molecules) and F is the fat magnetization (i.e. the magnetization caused by the fat molecules). Equations (1) and (2) can be converted into the following Equations (3) and (4) so that ultimately the water magnetization W and the fat magnetization F are determined.W=(M1+M2)/2  (3)F=(M1−M2)/2  (4)
According to the Dixon method, echoes can also be acquired at more than two different echo times in order to determine the water magnetization and the fat magnetization separately at these more than two magnetizations. Given the acquisition of more than two echoes, in addition to the two magnetizations additional effects (such as the transversal T2* relaxation) are also often considered and/or determined. In summary, the Dixon method determines the water magnetization and the fat magnetization (and possibly the transversal relaxation times T2*) per voxel based on at least two magnetizations which are determined or known for each voxel at different echo times, using the phase relationships of these magnetizations relative to one another (which phase relationships are known from the echo times).
However, the Dixon method also has weak points. For example, the Dixon method is susceptible to T1 relaxation effects. According to the prior art, this weak point is often counted via the use of RF excitation pulses with small flip angles (<10°), which must, however, be paid for in turn with a low signal-to-noise ratio.